Heretofore, measurement of the characteristics of smoke has been carried out by determining the transmissibility of the smoke with respect to directly transmitted light. In addition, measurement has been made of smoke characteristics by measuring transmissibility of reflected light. However, in order to carry out both measurements, two optical systems must be provided, one for each type of measurement. In this arrangement, either the light source for measuring the directly transmitted light characteristic or the one for measuring the reflected light characteristic must be turned off when the other is turned on, otherwise the light from the two sources interfere with each other. When measurements of the directly transmitted and reflected light characteristics are conducted repeatedly, therefore, the lamps constituting the respective light sources must be turned on and off repeatedly.
Moreover, the quantity of light emitted by a lamp is generally not the same immediately after it is turned on as it is after a certain lapse of time, so that the quantity of light in the above-described conventional apparatus varies corresondingly. For this reason, the values measured by the apparatus have inherently poor accuracy.